Unintentional injury is the leading cause of death among children, and contributes substantially to the burden of morbidity and disability experience in this population. The prevention of injuries is an important activity for health care providers and institutions. A clinical marker to identify families at experiencing periods of high child injury risk would be useful for guiding interventions. Medical encounters for injury care may be such a marker. Using data from a large health maintenance organization (HMO), the investigators propose a retrospective cohort study to confirm and generalize previous work that suggested a temporal association between family injury events and subsequent, additional childhood injuries. Enrollment and medical encounter data for 25,000 children and their families will be analyzed to characterize the association between an initial medically-treated childhood injury and the subsequent short-term risk of additional unintentional childhood injuries among siblings of the index case. Survival analysis with time-dependent covariates will be used to model the adjusted risk of injury as a function of time elapsed since the most recent, medically-attended sibling injury. Attributable risk estimates will also be produced to allow assessment of the potential impact of any proposed interventions. The long term goal of the research is to design and evaluate a time-limited, targeted, injury prevention program based upon the use of clinical markers to identify periods of increased injury vulnerability among families at risk. This intervention would likely be based upon the use of community-based social support resources for families under stress.